ReadOnlyAttribute

Example.cs

using UnityEngine;

public class Example : MonoBehaviour
{
    [SerializeFiled, ReadOnlyAttribute]
    private int value = 0;
    
    [SerializeFiled, ReadOnlyAttribute]
    private string[] array = null;
}

ReadOnlyAttribute.cs

using UnityEngine;

public class ReadOnlyAttribute : PropertyAttribute
{

}

ReadOnlyDrawer.cs

using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(ReadOnlyAttribute))]
public class ReadOnlyDrawer : PropertyDrawer
{
    public override float GetPropertyHeight (SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property, label, true);
    }

    public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
    {
        GUI.enabled = false;
        EditorGUI.PropertyField (position, property, label, true);
        GUI.enabled = true;
    }
}